This may or may not have been the best course of action, but I prioritized a bunch of other things ahead of Psionics research. Now that I have a Psi Lab, it apparently works the same way as the GTS, training Rookies to a new psi class.
There's just one problem: I'm all out of Rookies. When I go to pick up new recruits, at this point they all seem to be Sergeants or Lieutenants. Is there any way to get more Rookies so I can train them?